1. Field of the Invention
The present invention relates generally to broadband radio systems to which Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) is applicable, and more particularly to a base station, a mobile terminal, and a communications method capable of widening the band and increasing the efficiency of use of radio resources.
2. Description of the Related Art
In these years, broadband radio communications systems have been required to be broader in band and to increase the efficiency of use of radio resources to be cheaper per bit.
Examples of broadband radio communications systems include Worldwide Interoperability for Microwave Access (WiMAX) to which Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) is applied. WiMAX may be referred to as IEEE (Institute of Electrical and Electronic Engineers) 802.16e.
A description is given below of WiMAX as a broadband radio communications system to which OFDM/OFDMA is applied.
A description is given, with reference to FIG. 1, of the initial sequence of an initial network entry in a WiMAX system.
FIG. 1 is a flowchart showing an initial network entry procedure.
Referring to FIG. 1, a base station (BS) 10 puts DCD (Downlink Channel Descriptor)/UCD (Uplink Channel Descriptor) on a broadcast channel at any transmission intervals, and transmits DCD/UCD to each mobile terminal (mobile station: MS) 20 subordinate to the base station 10 (steps S2 and S6). In FIG. 1, the transmission interval is indicated by “Interval,” and is, for example, 10 s. Here, DCD is a medium access control (MAC) message that describes the physical layer characteristics of a downlink channel, and UCD is a medium access control (MAC) message that describes the physical layer characteristics of an uplink channel.
FIG. 2 is a diagram showing a transmission frame/reception frame configuration in WiMAX. (See, for example, IEEE 802.16e.) DCD/UCD provides information on the transmission frame/reception frame configuration. For example, DCD/UCD includes information such as TTG (Transmit/Receive Transition Gap)/RTG (Receive/Transmit Transition Gap), the uplink (UL)-downlink (DL) frame assignment ratio, and the modulation method of bursts for transmission/reception assigned to each of the uplink and downlink frames. Here, TTG indicates the gap of switching between transmission and reception, that is, the time interval between the transmission frame and the reception frame, and RTG indicates the gap of switching between reception and transmission, that is, the time interval between the reception frame and the transmission frame.
A description is given of the operation of the mobile terminal 20 being turned ON and entering a network.
Referring back to FIG. 1, in step S4, the mobile terminal 20 is turned ON to synchronize with a radio frame transmitted by the base station 10. For example, the mobile terminal 20 synchronizes with the radio frame by a preamble included in the radio frame. Then, the mobile terminal 20 reads a frame control header (FCH) to recognize a downlink map (DL-MAP). The DL-MAP includes information on the assignment of the downlink channel. Then, the mobile terminal 20 reads the DL-MAP to recognize the position of Burst #1. An uplink map (UL-MAP) and DCD/UCD have been transmitted in Burst #1. The mobile terminal 20 can read the UL-MAP and DCD/UCD by reading Burst #1.
As described above, the mobile terminal 20 understands the configuration of the entire frame by DCD/UCD, and reads the position of a ranging region set on the UL frame defined by the UL-MAP. Then, the mobile terminal 20 starts a network entry procedure (step S10) by transmitting a ranging code to the ranging region (step S8).
However, the above-described conventional technique has problems such as the following.
In radio communications systems to which OFDM/OFDMA is applied, a procedure called ranging should be followed as the network entry procedure of the mobile terminal 20. In order to recognize the ranging region included in the uplink subframe, which ranging region is necessary to proceed with the ranging, the mobile terminal 20 is required to understand the configuration of the entire frame by DCD/UCD and then read the position information of the ranging region defined on the UL-MAP.
However, UCD/DCD is large in data size. Further, the time interval of the broadcasting of DCD/UCD is an important parameter in system design. This time interval needs to be set at an optimum value based on the protocol overhead and the downlink transmission band. Currently, this time interval is considered to be approximately ten seconds.
For example, as shown in FIG. 3, in the case where DCD/UCD is broadcast (steps S2, S3, S6, S12, and S14), the mobile terminal 20 is turned ON (step S4), and the initial network entry is performed (step S11), ranging may be started after passage of the time interval of the broadcasting of DCD/UCD, for example, after ten seconds (step S8).